1. Field of the Invention
This invention relates to a plastic surrounded bearing comprising a bearing portion and a pulley portion made of plastic, and in particular to a plastic-surrounded bearing of a structure in which localized dimensional contraction is prevented after injection molding.
2. Description of the Prior Art
A so-called plastic-surrounded bearing in which a pulley made of plastic is integrally coupled to the outer race of the bearing by injection molding is well known and is generally used in a pulley or the like.
Practised as a method for manufacturing such plastic-surrounded bearing is, for example, a method whereby a finished bearing is pre-inserted into a plastic forming mold and molten plastic under high pressure is injection-poured into said forming mold from a nozzle through a gate to thereby accomplish molding, that is, the so-called insert molding method.
However, the pulley portion begins to cool from immediately after it is injected out of the nozzle, but the cooling progresses from the surface toward the interior of the pulley after the plastic has been poured into the forming mold and therefore, the portion of small thickness cools quickly and the portion of great thickness cools slowly. Accordingly, that portion of great thickness of the pulley portion which cools last after molding pulls the surrounding plastic when it solidifies and therefore, great localized dimensional contraction takes place in that portion of great thickness. Where the volume of the pulley portion is relatively great, if there is provided a plurality of gates and plastic is poured in therethrough, the plastic poured in through the respective gates forms a merging portion in a bisecting plane substantially equidistant from adjacent gates and weld lines are created in that portion.
In the portion of said weld lines, the plastic has already been considerably cooled and merged when the molding has been completed and therefore, as compared with the other portions, it is difficult for dimensional contraction to take place, and this portion results in a convex portion.
In the case of a use in which this convex portion is in contact with other part, for example, in a case where a belt moves over the outer peripheral surface of a pulley, the pulley may be damaged or vibrations may be induced and therefore, it becomes necessary to finish the outer peripheral surface of the pulley into a smooth surface. Also, said weld lines merge with one another after the plastic has been considerably cooled and therefore, they are weak in strength as compared with the other portions and may sometimes be reduced in strength as much as 70%.
FIG. 1 of the accompanying drawings illustrates the heretofore used pulley of plastic and a plastic pouring device for forming the same to eliminate the above-noted disadvantage peculiar to the well-known technique.
As shown in FIGS. 1 and 2 of the accompanying drawings, a pulley 6 made of plastic is made integral with the outer race 1 of a bearing B comprising an outer race 1, an inner race 2, balls 3, a retainer 4 and a seal 5, by insert molding. The pulley 6 has a main portion 61a radially surrounding the outer periphery of the outer race 1, and axially extended portions axially extending from said main portion 61a toward the opposite sides beyond the end surfaces of the outer race 1 and having the end surfaces of the outer race 1 embedded therein, the axially extended portions being integral with the main portion 61a.
Each of these axially extended portions has axially outwardly opening recesses 62a at circumferentially equally angled positions. That is, each extended portion has ring-like portions 60a and 60b extending toward the outer periphery side and the inner periphery side and an equally angled rib 63a radially connecting these two ring-like portions 60a and 60b and forming said recess 62a therebetween, and uniformizes the dimensional contraction of the pulley 6 as much as possible and provides moderate rigidity against the load from a belt or the like moving over the outer peripheral surface of the pulley.
As shown in FIG. 1, in a portion of the molten plastic pouring device for forming such pulley 6, there is a sprue S as indicated by dotted line at the center thereof, and five runners R1 extend radially from the sprue to uniformize the cooling as much as possible, and the tip end of each of the runners connects to a first gate Ga which provides the plastic inlet port of the pulley, said first gate being provided near the inner peripheral edge of the end surface portion of the pulley, namely, in the end surface portion of the ring-like portion 60b. The inner peripheral portion 60b of said extended portion juxtaposed with the end surface of the outer race 1 from the first gate Ga provides a ring-like runner R2. The radially outer portion of this ring-like runner R2 is the axially extended portion 60a of the pulley portion 61a. The ribs 63a between this ring-like runner and the axially extended portion 60a have also the function as a second gate which connects the ring-like runner R2 to the pulley portion 61a. These ribs as the second gate are circumferentially formed with an equal width and at an equal pitch, as previously described.
Regarding the recess 62a formed between adjacent ribs, it is desirable that the end surface of the outer race be exposed in order that plastic may preferably flow from the ring-like runner into the pulley portion only through the second gate.
Also, in the end surface of the pulley which is opposite to the gate Ga, recesses 62a and reinforcing ribs 63a similar to those on that side which is adjacent to the gate are formed to uniformize the dimensional contraction.
When molten plastic is poured in from said sprue to form a pulley of such shape, the flow of the molten plastic passes through the five runners R1 as indicated by arrows, and is poured into the first gates Ga, and the molten plastic flows chiefly circumferentially in the ring-like runner R2 and, each time the plastic arrives at the second gates 63a, it flows into the pulley portion 61a, and if, for example, one of the five first gates is called the gate a and the adjacent gate is called the gate b, the molten plastic poured in from the gate a and the plastic poured in from the gate b dividedly flow in two circumferential directions in the ring-like runner, and the dividedly flowing plastics pass through the ring-like runner and said second gates, whereafter in the pulley portion, they merge with each other in the bisecting plane Fa of the gates a and b, thereby forming a merging plane. This holds true of each first gate and thus, total five merging planes are formed. After its inflow, the plastic begins to cool with the lapse of time, and in the merging planes Fa, a reduction in pressure and temperature progresses as compared with the time of pouring in, but by a number of gates being provided, a great difference does not occur between the temperatures of the various portions and moreover, by providing the recesses, the thickness is uniformized as much as possible and therefore, creation of localized dimensional contraction and convex portions 64a harmful to the merging planes is reduced if not eliminated. However, if the number of the first gates is increased, the dimensional accuracy of the pulley portion will be correspondingly improved.
However, in the above-described example of the prior art, the presence of five first gates requires the presence of five runners, and this results in the problem of reduced yield of the material which in turn adversely affects the cost of the product.